Synthesis of organohalosilanes such as methylchlorosilane is industrially carried out by directly reacting organic halides such as alkyl halides or phenyl halides with metallic silicon particles in the presence of a copper catalyst at a temperature of 250 to 500° C., which is known as Rochow reaction. For this reaction, it is a key technology to produce the desired organohalosilane in high yields while keeping a high reaction rate. For example, in the case of methylchlorosilane synthesis, it is a key to increase the selectivity of dimethyldichlorosilane in the most demand, and in the case of phenylsilane synthesis, it is a key to produce desired diphenyldichlorosilane and phenyltrichlorosilane in a composition conforming to their demand. However, this reaction takes a long time for activation until a steady state is reached, and in turn, the steady state is relatively short so that the contact mass declines its activity with the passage of time, leading to a rapid drop of the yield of diorganodichlorosilane. For example, the synthesis of methylsilane often entails side reactions to increase a high-boiling fraction such as disilanes, methyltrichlorosilane and other unwanted products, which requires premature replacement of the contact mass within the reactor and removal of scale. It is known that these problems can be solved by effecting reaction in the presence of various co-catalysts such as zinc, tin and phosphorus. With respect to the co-catalysts, while too much emphasis is made on their positive functions, little study has been made on their negative functions. The current practice is merely to change, on an empirical basis, the composition of additional contact mass with the progress of reaction.
Making a close study on the action mechanism of the co-catalyst used in Rochow reaction and the behavior thereof within a reactor, the inventor attempted to optimize the form of the co-catalyst. An object of the invention is to provide a co-catalyst for use in organohalosilane synthesis which can reduce the activation time taken until a steady state is reached, which has been an outstanding problem in Rochow reaction, and increase the selectivity of desired diorganodihalosilane for eventually improving the reaction results. Another object of the invention is to provide a process for preparing organohalosilanes using the co-catalyst.